Parathyroid venous sampling remains the most precise localization technique for patients with failed parathyroid surgery but it requires considerable experience on the part of the angiographer as well as prolonged catheter manipulation to sample the multiple small veins draining the neck and mediastinum. Knowledge of the presence of parathyroid hormone (PTH) gradients during the procedure would enable the angiographer to focus on a specific area, perhaps obtaining more detailed samples and to shorten the procedure when significant gradients occur in the early samples. This protocol will not increase the accuracy of sampling but will shorten the procedure, reduce radiation exposure to the patient and to the interventional radiologist and allow more precise localization by more focused sampling.We plan to use the quick PTH assay originally developed by Nichols? Lab for intraoperative use to inform the surgeon when all abnormal parathyroid tissue has been resected. However as we would be assaying multiple PTH samples on line and the assay is not cheap, we have been collaborating with Nichols to develop a more sensitive, less expensive rapid PTH assay. This has recently been accomplished and Nichols? Lab has agreed to provide assay kits at cost for our investigation. Although no patients have been accrued to this protocol thus far, on-line PTH assays during parathyroid venous sampling will be a useful clinical addition to the work-up of patients with occult parathyroid adenomas and we plan to continue this protocol.